Hitherto, as wiring materials for display devices such as flat panel displays, there have been generally used aluminum and aluminum alloys. However, with the recent increase in size and resolution of the displays, the aluminum-based wiring materials tend to suffer from problems such as signal delay owing to their properties such as a wiring resistance, and therefore tend to hardly provide a uniform image display.
In consequence, studies have been made to provide wirings formed of a lower-resistance material such as copper and a material containing cooper as a main component. However, copper is advantageous in its low resistance, but has problems such as poor adhesion to a substrate such as a glass substrate when used in gate wirings and occurrence of undesirable diffusion to an underlying silicon semiconductor film when used in source/drain wirings. In order to avoid these problems, there have been made studies on lamination of a barrier film containing a metal having a barrier property capable of preventing diffusion of copper, etc., to the silicon semiconductor film. As the metal used in the barrier film, titanium (Ti) and molybdenum (Mo) have been studied, and a multilayer thin film using copper in combination with these metals has been proposed.
The multilayer thin film containing copper or a copper alloy containing copper as a main component is formed on a substrate such as a glass substrate by a film-forming process such as sputtering. Then, after masking a resist applied onto the multilayer thin film or the like, the film is subjected to an etching step to form an electrode pattern thereon. The etching step may be conducted either by a wet method using an etching solution or by a dry method using an etching gas such as plasma. The etching solution used in the wet method is required to provide (i) a high processing accuracy, (ii) a less amount of etching residues, (iii) a high stability or safety of its components with easiness of handling, (iv) a stable etching performance, etc.
As the etching solutions conventionally used for etching the multilayer thin film containing copper or a copper alloy containing copper as a main component, there are known an etching solution containing at least one substance selected from the group consisting of hydrogen peroxide, a neutral salt, an inorganic acid and an organic acid (refer to Patent Document 1), an etching solution containing hydrogen peroxide, a carboxylic acid and fluorine (refer to Patent Document 2), an etching solution containing hydrogen peroxide, an organic acid, a phosphoric acid salt, a nitrogen-containing first additive, a second additive and a fluorine compound (refer to Patent Document 3), an etching solution containing hydrogen peroxide, a fluorine ion supply source, a sulfuric acid salt, a phosphoric acid salt and an azole-based compound (refer to Patent Document 4) or the like.
However, these etching solutions containing hydrogen peroxide have problems such as (i) large change in etching rate owing to change in concentration of hydrogen peroxide, (ii) generation of gases and heat owing to abrupt decomposition of hydrogen peroxide which tends to cause a risk of breakage of facilities, or the like. For these reasons, there is an increasing demand for an etching solution containing no hydrogen peroxide.
On the other hand, as the etching solution containing no hydrogen peroxide, there are known ammoniacally alkaline etching solutions containing a copper (II) ion and ammonia (for example, refer to Patent Document 5 and Non-Patent Documents 1 and 2). Even these ammoniacally alkaline etching solutions are capable of etching a multilayer thin film containing copper or a copper alloy containing copper as a main component such as copper/molybdenum-based multilayer films. However, the etching solutions tend to suffer from volatilization of a large amount of ammonia therefrom owing to a high pH value of the solutions and therefore reduction in ammonia concentration in the solutions. As a result, there tend to occur variation in etching rate and considerable deterioration in working environments. In addition, the high pH value of the etching solutions tends to pose such a problem that a resist is dissolved in the solutions. In this case, volatilization of ammonia from the etching solutions may be suppressed by adjusting the pH value thereof to a neutral range. However, the neutral etching solutions have problems such as deposition of residues upon rinsing with water after etched.